1. Field of the Invention
This invention relates to the self testing of micromachined silicon structures in general and to the method and apparatus for thermally actuated self testing of a silicon flexure strain indicator.
2. Description of the Prior Art
Many micromechanical silicon devices are now well known, including sensors for sensing force, pressure, acceleration, chemical concentration, etc. Such devices are termed "micromechanical" because of their small dimensions on the order of a few millimeters square. Such sizes are achieved by utilizing a photolithographic technique similar to that employed in the fabrication of integrated circuits. Silicon wafers well known in the integrated circuit technology can also be used for micromachined structural elements and have the advantage that additional control or sensing electronic circuitry can be formed in conjunction with the structure providing the sensing, in order to process the resultant electrical signal.
Micromachined silicon is well known in a number of different applications. Many operational devices depend upon the flexing of a thin area of silicon which connects relatively thicker areas of unconnected silicon, the so-called "boss" areas. For example, in a pressure transducer, there may be a large, relatively stiff, silicon diaphragm boss bounded by a thin, relatively flexible area along its periphery which permits the diaphragm to move and the thin area to flex depending upon pressure differentials on the sides of the diaphragm. A detailed discussion of the use of relatively thin flexure areas and relatively thick boss areas for pressure or other type silicon sensors is contained in "Low Pressure Sensors Employing Bossed Diaphragms and Precision Etch-Stopping" by Mallon, Barth, Pourahmadi, Vermeulin, Petersen and Bryzek published Jun. 25, 1989 herein incorporated by reference.
Silicon machined flexure areas are also used in accelerometers and the general concept is disclosed in U.S. Pat. No. 4,882,933 issued to Petersen et al on Nov. 28, 1989 also herein incorporated by reference. Common to both pressure sensors and accelerometers is the use of an impurity doped portion of the flexure area which changes resistance (generates a potential) depending upon the amount of flexure developed (piezoresistive effect).
The use of accelerometers, and particularly silicon accelerometers, in the automotive field has increased dramatically in the last several years. Market forces have encouraged development of low cost batch fabricated accelerometers. Primary among the automotive uses are for crash sensors to control driver and passenger air-bag deployment as well as ride motion sensors for active suspension systems and automatic braking systems (ABS). With respect to crash sensors, al acceleration sensor with full scale range of 25 to 100 g's can be used to detect an automobile collision and to determine whether or not a protective air-bag should be inflated. For ride motion sensors, sensitivity for relatively low accelerations is necessary with accelerations being in the range of 0.5 to 2 g.
As discussed in the '933 patent, silicon sensors are rather brittle and the breakage problem can reduce the batch yield for such sensors. Generally in silicon accelerometers, a seismic mass is connected to the supporting frame of the chip by a flexible beam such that stress sensitive resistors located on the beam measure the mass' deflection and therefore the acceleration of the chip itself. The '933 discusses an accelerometer embodiment with bidirectional shock protection, as well as controllable viscous damping.
However, whether crash sensing accelerometers in the range of 25 to 100 g's are concerned or ride motion sensor accelerometers in the range of 0 to 2 g are needed, there exists no low cost method of testing the operational accuracy of such accelerometers. Additionally, there is also no low cost method of testing the operability and/or accuracy of silicon pressure sensors.
In the past, a relatively expensive method of testing was to provide conductive portions on the seismic mass and the supporting chip and a substantial voltage potential is applied to the two portions. The electrostatic attraction deflects the seismic mass in the normal flexure direction simulating an acceleration applied to the accelerometer. Unfortunately, relatively high voltages on the order of 50 volts or more are required to simulate a 50 g acceleration force and such voltage levels are difficult and/or expensive to generate, especially at low cost for application in an automotive environment.